Inhibitor of Apoptosis (IAPs) proteins are endogenous inhibitors of programmed cell death. Elevated levels of IAPs are often found in cancers, due to gene amplification, translocation, or overexpression (Fulda and Vucic, 2012). In vitro and in vivo studies have associated high IAP levels with tumorigenesis, chemoresistance, disease progression, and poor prognosis (Hess et al., 2007; Nakagawa et al., 2006; Ramp et al., 2004; Tamm et al., 2004). The role of IAPs in regulating apoptosis, and the correlation of IAP expression with cancer progression provided the rationale for the design and development of IAP antagonist compounds (also known as “SMAC mimetics”), a new class of cancer therapeutic.
The key IAPs-XIAP, cIAP1 and cIAP2-bear three tandem Baculoviral IAP Repeat domains (BIRs) and a C-terminal E3 ligase RING domain. SMAC/Diablo is a natural IAP antagonist protein that when released from the mitochondria binds to IAPs (Du et al., 2000; Verhagen et al., 2000). SMAC mimetics are modeled on the N-terminal AVPI tetrapeptide of SMAC/Diablo and bind to the second and third BIR domain of IAPs. The development of SMAC mimetics has helped to define the exact role of IAPs in the regulation of cell death pathways. XIAP exhibits its anti-apoptotic activity through direct binding to and inhibition of caspase-3, -7 and -9 (Srinivasula et al., 2001). cIAP1 and cIAP2 on the other hand are not direct caspase inhibitors, but regulate caspase activation indirectly through their E3 ligase activity (Bertrand et al., 2008; Eckelman and Salvesen, 2006; Vince et al., 2007). In response to TNFα, cIAP1 and cIAP2 ubiquitylate RIPK1 to lessen its ability to activate FADD and caspase-8 (Bertrand et al., 2008; Feoktistova et al., 2011). Within the TNFα/TNFR1 complex cIAPs promote activation of p65/RelA NF-κB, whereas in the cytosol they constitutively ubiquitylate NIK to reduce signaling by p100/p52 NF-κB2 (Feltham et al., 2011; Zarnegar et al., 2008).
Consequently, SMAC mimetics trigger apoptotic cell death in cancer cells in two ways. Firstly, by binding to the BIRs of cIAP1 and cIAP2 they trigger a conformational change which allows the RING domains to dimerise, activating their E3 ligase activity, leading to cIAP1 and cIAP2 auto-ubiquitylation and subsequent proteasomal degradation (Dueber et al., 2011; Feltham et al., 2010). cIAP1 and 2 degradation induces the activation of p52 NF-κB2 leading to synthesis of TNFα. The autocrine secretion of TNFα activates TNFR1 signaling that, in the absence of cIAP1 and 2, results in the activation of caspase-8 (Feoktistova et al., 2011). Secondly, binding of SMAC mimetics to XIAP's BIR domains prevents XIAP from directly binding to and inhibiting caspases-3, -7 and -9.
In vitro and in vivo studies have demonstrated that SMAC mimetics have excellent anti-tumour activity as single agents in several cancer cell lines (Fulda and Vucic, 2012). Moreover when combined with some conventional chemotherapeutic agents, SMAC mimetics display synergistic effects (Fulda and Vucic, 2012) Importantly, administration of SMAC mimetics showed no significant toxicity in mouse xenograft models (Fulda and Vucic, 2012). Based on the positive results from preclinical studies, several SMAC mimetics have completed phase I safety studies in the clinic. Amongst them, the TetraLogic compound called Birinapant (TL32711) is the most advanced. A Phase I study in patients with advanced solid tumours and lymphoma showed that intravenous administration was well tolerated with no dose-limiting toxicities and demonstrated evidence of anti-tumour activity (Amaravadi et al Poster abstract #2532. 102nd AACR annual meeting. 2011, Orlando, Fla.). A Phase Ib/IIa five-arm study of Birinapant in combination with four different chemotherapies is currently under way (ClinicalTrial.gov, NCT01188499). Additional Phase I and II clinical studies are recruiting for both solid tumours and hematological malignancies (ClinicalTrial.gov, NCT01573780, NCT01486784).
The p38 pathway regulates key cellular signaling related to inflammation. For instance, upon Toll Like Receptor (TLR) activation, p38 induces a phosphorylation cascade involving kinases and transcription factors leading to TNFα transcription and secretion (Gaestel, 2013). The kinase MK2 has been demonstrated to be the downstream mediator of p38 that serves to activate TNFα transcription in response to TLRs (Gaestel, 2013). Its crucial role in inflammatory cytokine expression has been exploited to develop p38 inhibitors to inhibit TNFα production in auto-immune diseases. To date, several orally active p38 inhibitors have entered clinical trials (Cohen and Alessi, 2013). However, despite encouraging preclinical findings, none of these compounds advanced to late stage clinical trials due to lack of efficacy and/or unacceptable side effects (Cohen and Alessi, 2013). In recent years, several potential reasons have come to light to explain why p38 may not be an optimal target for the development of anti-inflammatory drugs. For example, p38 participates in feedback control loops that suppress the activities of ‘upstream’ MAPK, implicated in the activation of the MAPK JNKs, which stimulate the production of TNFα during inflammatory processes. Therefore, drugs that inhibit p38 cancel these feedback control loops, leading to a hyperactivation of JNKs and a consequent increase in pro-inflammatory cytokines that may contribute to the modest clinical responses and hepatic toxicities (Cohen and Alessi, 2013).